Pressure transducers normally include pressure sensor headers. See for example, U.S. Pat. No. 4,695,817 entitled, ENVIRONMENTALLY PROTECTED PRESSURE TRANSDUCERS EMPLOYING TWO ELECTRICALLY INTERCONNECTED TRANSDUCER ARRAYS issued to A. D. Kurtz et al. on Sep. 22, 1987 and assigned to the assignee herein. Certain pressure sensor headers include a metal header shell that defines a sensor cavity. The sensor cavity houses a sensor element and contains a fluid medium, which at least covers the sensor. The sensor cavity is hermetically sealed with an isolation diaphragm that is welded to the header shell.
In normal operation, the fluid medium transmits pressure from the isolation diaphragm to the silicon sensing diaphragm of the sensor. Silicone oil is usually selected as the fluid medium because it exhibits minimum compressibility, and thus, allows accurate transmission of pressure without nonlinearities or dead-bands.
Pressure transducers may be employed in high pressure environments. For example, pressure transducers are used for monitoring pressure in power generating pumps. The pressure sensor headers of these transducers often operate under external (hydrostatic) pressures, which can reach extremes, up to and in excess of 50,000 psi. These pressures act on the front face and side wall of the header. The pressure force acting on the header shell's cylindrical side wall generates compressive tangential and radial stresses (hoop stress) in the side wall. Although pressure sensor headers can be helium leak tested and qualify as hermetic, such sensors used in the presence of hydrogen gas can leak, allowing the introduction of hydrogen into the oil-filled sensor cavity, as the hydrogen molecules are much smaller than helium molecules.
A possible entry path for the hydrogen is the isolation diaphragm weld. Conventional designs employ a laser welding process to hermetically seal the isolation diaphragm to the header shell. In this welding process, the diaphragm is positioned on the header shell and spot welded. The diaphragm is then fully laser welded to the header shell using a conventional lap or partially penetrating weld. Under excessive cyclic operation, the weld area experiences high stress, which tends to cause the propagation of very small cracks in these shallow, conventional lap or partially penetrated welds joining the isolation diaphragm with the header shell. The stress may result in weld fracture and fatigue failure, thus, presenting an entry point for the hydrogen gas molecules, which are much smaller than helium gas molecules. Thus, over time, the hydrogen gas can penetrate the very small cracks in the shallow, conventional welds.
When hydrogen gas is introduced into the oil-filled sensor cavity, the system pressure must first compress the hydrogen gas, before transmitting the pressure through the silicone oil. This gas compression presents a dead-band at low pressures, and causes a non linear effect on the sensor output.
Thus, a pressure sensor header is needed which has an integrated isolation diaphragm and diaphragm/header shell weld area that prevents entry of hydrogen gas into the sensor cavity under excessive cyclic operation in extreme external pressures.